vCenter Single Sign-On includes the Security Token Service (STS), an administration server, and vCenter Lookup Service, as well as the VMware Directory Service (vmdir). The VMware Directory Service is also used for certificate management.

During installation, the components are deployed as part an embedded deployment, or as part of the Platform Services Controller.

STS (Security Token Service)
The STS service issues Security Assertion Markup Language (SAML) tokens. These security tokens represent the identity of a user in one of the identity source types supported by vCenter Single Sign-On. The SAML tokens allow both human users and solution users who authenticate successfully to vCenter Single Sign-On to use any vCenter service that vCenter Single Sign-On supports without authenticating again to each service.
The vCenter Single Sign-On service signs all tokens with a signing certificate, and stores the token signing certificate on disk. The certificate for the service itself is also stored on disk.
Administration server
The administration server allows users with administrator privileges to vCenter Single Sign-On to configure the vCenter Single Sign-On server and manage users and groups from the vSphere Web Client. Initially, only the user administrator@ your_domain_name has these privileges. In vSphere 5.5, this user was administrator@vsphere.local. With vSphere 6.0, you can change the vSphere domain when you install vCenter Server or deploy the vCenter Server Appliance with a new Platform Services Controller. Do not name the domain name with your Microsoft Active Directory or OpenLDAP domain name.
VMware Directory Service (vmdir)

The VMware Directory service (vmdir) is associated with the domain you specify during installation and is included in each embedded deployment and on each Platform Services Controller. This service is a multi-tenanted, multi-mastered directory service that makes an LDAP directory available on port 389. The service still uses port 11711 for backward compatibility with vSphere 5.5 and earlier systems.

If your environment includes more than one instance of the Platform Services Controller, an update of vmdir content in one vmdir instance is propagated to all other instances of vmdir.

Starting with vSphere 6.0, the VMware Directory Service stores not only vCenter Single Sign-On information but also certificate information.

Identity Management Service
Handles identity sources and STS authentication requests.